This invention relates to a base station transmitter-receiver for repeating transmission and reception modes, transmitting a signal in the transmission mode and receiving a signal in the reception mode. More particularly, the invention relates to a transmitter-receiver of a base station for mobile communications utilizing TDD (time-division duplexing) in a system such as the PHS (Personal Handyphone System).
In order to reduce the load on hardware on the terminal side, achieve a reduction in size and weight and assure a long continuous standby time, the PHS implements two-way transmission of information by TDD.
TDD is duplexing that is accomplished by alternately transmitting and receiving signals using different time intervals for each signal. For example, in the case of PHS, transmitting-receiving timing in 4-channel multiplexed TDMA-TDD is as shown in FIG. 23. In one frame equivalent to 5 ms, four transmission slots (CH1 TX.about.CH4 TX) and four reception slots (CH1 RX.about.CH4 RX) of 625 .mu.s are exchanged. In FIG. 23, CS (Cell Station) represents a base station, PS1.about.PS4 (PS: Personal Station) represent mobile slave stations, TS represents the duration of the transmission mode of the base station, and RS represents the duration of the reception mode of the base station. The base station CS transmits information to the four slave stations PS1.about.PS4 for 5 ms in the form of burst signals each having a duration of 625 .mu.s. Each of the slave stations PS1.about.PS4 transmits its own information by a 625-.mu.s burst 2.5 ms after receiving the signal from the base station CS.
When a number of PHS providers deploy base stations close together in the same area, as has happened recently, interference can result in transmission and reception. The reason for this is that there is no synchronization of frames between providers. Consequently, when base stations are set up independently, the transmission slots of the base station of a provider A can overlap the reception slots of the base station of a provider B, thereby impeding the reception of radio waves from the slave stations of provider B.
The simplest method of solving this problem is for the two base stations to transmit frame synchronizing signals to each other via a cable, with each station changing over transmission and reception timing in conformity with the frame synchronizing signal received. However, base stations usually are deployed independently of one another and the work involved in connecting base stations together is not easy even when the base stations are located in adjoining buildings. This approach, therefore, is not realistic. If it is attempted to transmit the frame synchronizing signal using ordinary lines, this in itself will result in the construction of a synchronizing network. This expedient is not economical.
A method referred to as "air synchronization" is used as one expedient to avoid the above-mentioned problem involving transmission of the synchronizing signal via cable. According to the air synchronization method, a base station that goes into operation at a later time receives radio waves from base stations that have already set up operation and begins operating in sync with these radio waves.
In the process of establishing synchronization of transmission and reception timing by air synchronization, it is required that operation different from the ordinary be performed, namely that a base station receive the radio waves that are emitted by another base station. A problem that arises is an excessively large input to the receiver of the base station. Since a base station produces a larger transmission output than a slave station, radio waves emitted from a base station a short distance away represent an excessively large input to the receiving base station. The result is a deterioration in the characteristic of the front-end low-noise amplifier, and the amplifier can be damaged in extreme cases. Since constructing a low-noise amplifier using devices that can withstand large inputs would invite a rise in the noise characteristic and lower reception sensitivity, this is not an effective solution to the problem. The problem set forth above will be confronted increasingly in situations where different PHS providers deploy their base stations in adjacent buildings or where a plurality of base stations are set up at one location in order to increase the number of subscribers by making use of a WLL (Wireless Local Loop).